Whenever a raider meets another minion or boss surge occurs. You will only have to fight 1 raider or boss on a hex. The rules say surges always happen on the end of a phase, so in a rare case a player could use a instant action card to push an enemy away during the encounter phase, thus making it possible for 2 enemies to be on the same hex during the next players encounter.
Answer: In that case you will first fight one raider (your choice) and IF you lose the battle and the raider survive because there will be 2 raiders on the same Hex a surge will happen.
Some cards allow you to kill enemies outside the normal encounter phase, like one of the player cards that allows a player (paying a combat token) to move 1 sector and fight a raider. (So think this combat just happens in the movement phase). Or one of the advanced action cards that allows to throw a dice for every raider 1 sector away, destroying it on a 5 or 6. Do we get rewards for kills like these?
Answer: Whenever you make an extra combat you get extra rewards, not whenever you kill an enemy.
When you engage a combat against a raider, do you fight until death or can you stop the fight before getting killed (retreat)?
Answer: You fight until death